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Full text of "Solar Forcing of Regional Climate Change During the Maunder Minimum"

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Solar Forcing of Regional 

Climate Change During the 

Maunder Minimum 

Drew T. Shindell,^ Gavin A. Schmidt.^ Michael E. I^ann,^ 
David Rlnd,^ Anne Waple^ 

We examine the climate response to solar irradiance changes between the late 
17th century Maunder Minimum and the late 18th century. Global average 
temperature changes are small (about 0.3 to 0.4**C) in both a climate model and 
empirical reconstructions. However, regional temperature changes are quite 
large. In the model, these occur primarily through a forced shift toward the low 
index state of the Arctic Oscillation/North Atlantic Oscillation. This leads to 
colder temperatures over the Northern Hemisphere continents, especially in 
winter (1 to 2**C), in agreement with historical records and proxy data for 
surface temperatures. 



A minimum in solar irradiance, the Maunder 
Minimum, is thought to have occurred from 
the mid- 1600s to the early 1700s (1-3). Con- 
currently, surface temperatures appear to 
have been at or near their lowest values of the 
past millenium in the Northern Hemisphere 
(NH) (4-7), and European winter tempera- 
tures were reduced by 1 to 1.5°C (S). 

We used a version of the Goddard Insti- 
tute for Space Studies (GISS) general circu- 
lation model (GCM), which includes a de- 
tailed representation of the stratosphere, to 
simulate the difference between that period 
and a century later, when solar output re- 
mained relatively high over several decades. 
The model contains a mixed-layer ocean, al- 
lowing sea surface temperatures (SSTs) to 
respond to radiative forcing, and has been 
shown to capture observed wintertime solar 
cycle-induced variations reasonably well (P). 

The GCM includes parameterizations of 
the response of ozone to solar irradiance, 
temperature, and circulation changes (al- 
though transport changes are noninteractive) 
(9), based on results for preindustrial condi- 
tions from our two-dimensional (2D) chem- 
istry model (W). These show more ozone in 
the upper and lower stratosphere, as expected 
(II), primarily because of the absence of 
anthropogenic halogens and a drier strato- 
sphere (12). Ozone's temperature sensitivity 
was also increased throughout the upper 
stratosphere, more than doubling at 3 milli- 
bars (mbar) relative to the present (13). Solar 
heating then speeds up chemical destmction 
more than irradiance enhances photolytic 
production, so that ozone concentration de- 

^NASA Goddard Institute for Space Studies and Cen- 
ter for Climate Systems Research, Columbia Univer- 
sity. New York, NY 10025. USA. ^Department of 
Environmental Sciences, University of Virginia, Char- 
lottesville. VA 22902, USA. ^Department of Ceo- 
sciences, University of Massachusetts. Amherst, MA 
01003. USA 



creases with solar output. Thus, the reduced 
Maunder Minimum irradiance leads to in- 
creased upper stratospheric ozone, which in 
turn causes lower stratospheric ozone to de- 
crease. Both cause negative radiative forcing 
(14). Inclusion of ozone photochemistry 
therefore enhances radiative forcing from 
preindustrial solar variations. 

Equilibrium simulations were performed 
for spectrally discriminated irradiances in 
1680 and 1780 (I). An alternative reconstmc- 
tion has a 40% larger long-term solar varia- 
tion, which would give a larger climate re- 
sponse (15). Initial conditions were taken 
from 1 680 and 1780 in a transient simulation. 
A spin-up time of about 25 years was never- 
theless required for equilibration with the 
increased preindustrial ozone. Results are 
therefore based on the past 30 years of 60- 
year simulations. 

Modeled global annual average surface 
temperatures were 0.34°C cooler in 1680 
than in 1780. This is similar to the annual 
average NH 1680-minus-1780 temperature 
change of about -0.2 ± 0.2^C from proxy 
data (6) or an energy balance model (7) 
(model NH cooling is -0.33 C). Other proxy- 
based reconstructions for the NH extratropics 
during summer indicate a -0.3 to -0.4X 
cooling (4,5) (comparable model cooling is 
-0.3°C). 

Modeled surface temperature changes 
show alternating warm oceans and cold con- 
tinents at NH mid-latitudes (Fig. 1), with 
maximum amplitude in winter. To character- 
ize regional changes, we calculated empirical 
orthogonal functions (EOFs) from the NH 
extratropical (20° to 90°N) cold season (from 
November to April) sea level pressure (SLP) 
time series. The last 40 years of each nin 
were used as an 80- year control. 

The leading control EOF (Fig. 2) closely 
matches the Arctic Oscillation pattern (AO, 
also called the Northern Hemisphere Annular 



Mode), defined as the first EOF of 20th- 
century SLP (16). This pattern contains the 
North Atlantic Oscillation (NAO), which 
may be considered a different view of the 
same phenomenon (7 7). We define the mod- 
el's AO pattern as EOF 1 , and the AO index 
as the opposite of the area-weighted change 
in SLP northward of 60°. Projection of the 
1780-to-1680 SLP difference (Fig. 2) onto 
the first five control EOFs shows that they 
account for 37.3, 0.0, 3.4, 1.5, and 0.0% of 
the variance, respectively. Thus, the regional 
climate variations primarily result from a 
lower index state of the AO during the Maun- 
der Minimum relative to that of a century 
later (by -1.1 mbar; similariy, the SLP con- 
trast between Iceland and the Azores decreas- 
es by 1 .9 mbar). This reduces onshore advec- 
tion of warm oceanic air, yielding a 1° to 2°C 
cooling over large areas of North America 
and Eurasia (Fig. 1 ). 

Historical data associate the European 
Maunder Minimum winter cooling with en- 
hanced northeasteriy advection of continental 
air, consistent with an anomalous negative 
NAO (18). Reconstructions of large-scale 
surface temperature patterns in past centuries 
(19, 20) from networks of diverse proxy in- 
formation (tree rings, ice cores, corals, and 
historical records) are particulariy useful 
benchmarks for comparison. Correlations be- 
tween these reconstmctions and solar irradi- 
ance reconstmctions (1) during the preanthro- 
pogenic interval from 1 650 to 1 850 provide 
an empirical estimate of the large-scale cli- 
mate response to solar forcing (21). The in- 
stantaneous (zero-lag) frequency-indepen- 
dent regression pattern, primarily indicative 
of decadal solar-climate relationships, shows 
little evidence of an AO/NAO signal (Fig. 3, 
top). TTiis is consistent with the extremely 
weak modeled AO/NAO response to the 11- 
year solar cycle using fixed SSTs, equivalent 
to an instantaneous response (22). However, 
when filtered to isolate the multidecadal/cen- 
tennial time scales associated with the Maun- 
der Minimum, and lagged to allow for inertia 
in the ocean's response, the empirical regres- 
sion (Fig. 3, bottom) shows a clear AO/NAO- 
type pattern of alternating cold land and 
warm ocean temperature anomalies. Both pa- 
leoclimate reconstmctions and the GCM thus 
indicate in a remarkably consistent manner 
that solar forcing affects regional scales much 
more strongly than global or hemispheric 
scales through forcing of the AO/NAO. 

Most additional paleoclimate data have 
insufficient time resolution to distinguish the 
Maunder Minimum but suggest a shift toward 
the AO/NAO low-index state during periods 
of reduced solar forcing. Ocean sediments 
show a decreased SST of I to 2°C in the 
Sargasso Sea during the 16th to 19th centu- 
ries, and warmer Atlantic temperatures north 
of 44°, extending to the area off Newfound- 



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Annual Temperature change (C) 



-1.75 -.7 -5 -.35 -.2 -.05 .05 .2 .35 .5 .7 .9 

Winter Temperature change (C) 

Fig. 1. Surface temperature change (in °C) from 1780 to 1680 in the GCM. Global annual average (left) and November to April NH extratropics (right) 
are shown. Nearly all points are statistically significant (not shown) because of the large number of model years. Data for all figures are available as 
online supplemental material [40]. 




I I 



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-.8 -.3 .3 .8 1.4 1.9 2.5 

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Fig. 2. The leading EOF of NH extratropical November-to -April SLR over the past 40 years of the 
1680 and 1780 simulations (left), and the SLP change from 1780 to 1680 (right), both in mbar 
(mb). The SLP difference is filtered In EOF space by showing the projection onto the first 20 EOFs, 
which contain 70% of the total difference. This removes some of the high-frequency noise in the 
model induced by overly large energies at high wavenumbers. Hatched areas Indicate statistical 
significance at the 90% level. 



land (23\ which is consistent with a reduced 
NAG but not with uniform basin- wide cool- 
ing. Other evidence suggests colder North 
Atlantic temperatures during those centuries, 
however, including evidence for increased 
sea ice around Iceland (24 ), a region of min- 
imal change in the GCM. 

Our previous studies have demonstrated 
how external forcings can excite the AO/ 
NAO in the GISS GCM (22, 25). Briefly, the 
mechanism works as follows, using a shif^ 
toward the high-index AO/NAO as an exam- 
ple: (i) tropical and subtropical SSTs warm, 
leading to (ii) a warmer tropical and subtrop- 
ical upper troposphere via moist convective 



processes. This results in (iii) an increased 
latitudinal temperature gradient at around 1 00 
to 200 mbar, because these pressures are in 
the stratosphere at higher latitudes, and so do 
not feel the surface warming (26). The tem- 
perature gradient leads to (iv) enhanced lower 
stratospheric westerly winds, which (v) re- 
fract upward, propagating tropospheric plan- 
etary waves equatorward This causes (vi) 
increased angular momentum transport to 
high latitudes and enhanced tropospheric 
westerlies, and the associated temperature 
and pressure changes corresponding to a high 
AO/NAO index. Observations support a 
planetary wave modulation of the AO/NAO 



(27, 28), and zonal wind and planetary wave 
propagation changes over recent decades are 
well reproduced in the model (22). 

Reduced irradiance during the Maunder 
Minimum causes a shift toward the low-index 
AO/NAO state via this same mechanism. 
During December to February, the surface 
cools by 0.4° to 0.5°C because of reduced 
incoming radiation and the upper stratospher- 
ic ozone increase. Cooling in the tropical and 
subtropical upper troposphere is even more 
pronounced (~0.8°C) because of cloud feed- 
backs, including an —0.5% decrease in high 
cloud cover induced by ozone through sur- 
face effects. A similar response was seen in 
simulations with a finer resolution version of 
the GISS GCM (J 4). This cooling signifi- 
cantly reduces the latitudinal temperature 
gradient in the tropopause region, decreasing 
the zonal wind there at ~40°N. Planetary 
waves coming up from the surface at mid- 
latitudes, which are especially abundant dur- 
ing winter, are then deflected toward the 
equator less than before (equatorward Elias- 
sen-Palm flux is reduced by 0.41 m^/s^, 12° 
to 35°N, 300 to 100 mbar average), instead 
propagating up into the stratosphere (in- 
creased vertical flux of 6.3 X 10""* m^/s^, 35° 
to 60°N, 100 to 5 mbar average) (29). This 
increases the wave-driven stratospheric resid- 
ual circulation, which warms the polar lower 
stratosphere (up to 1°C), providing a positive 
feedback by fiirther weakening the latitudinal 
temperature gradient. The wave propagation 
changes imply a reduction in northward an- 
gular momentum transport, hence a slowing 
down of the middle- and high-latitude west- 
erlies and a shift toward the low AO/NAO 
index. Because the oceans are relatively 
warm during the winter owing to their large 
heat capacity, the diminished flow creates a 



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Temperature change (C) 

Fig. 3. Regression during the period from 1650 to 1850 between reconstructed solar in-adiance (7) 
and annual average surface temperatures (79). The top panel shows the instantaneous regression 
over all time scales, whereas the bottom panel shows the regression when filtered to only include 
contributions from time scales longer than 40 years, with a 20-year lac (results are similar for time 
lags of 10 to 30 years). The con-elation is given in °C per -0.32 W/m^ (the solar forcing used in the 
model) for comparison with the modeled temperature changes shown in Fig. 1. Gray areas indicate 
no data; hatched areas indicate statistical significance at the 90% level. 



cold-land/warm-ocean surface pattern (Fig. 
1). 

To quantify the importance of ozone 
changes, an identical pair of 1680 and 1780 
experiments was run, leaving out the ozone 
response. These simulations show a reduction 
in the AO index of only 0.5 mbar, and the 
SLP change prDJection onto the AO was 
15.1%. Thus, there was an analogous but 
weaker response without interactive ozone. 
The ozone changes flirther reduce the tropo- 
pause radiative forcing by -0.02 W/m^ (in- 
stantaneous, but with adjusted stratospheric 
temperatures) for 1680 conditions (compared 
to the direct solar forcing of -0.32 W/m^). 
Qoud feedbacks caused by ozone magnify its 
influence somewhat, so that the global annual 
average surface temperature change increases 
from -0.28°C to -0.34°C, including ozone 
chemistry. The largest impacts resulted fh)m 
the enhancement of the tropopause region 
latitudinal temperature gradient through high 
cloud changes and subsequent wave-driven 
feedbacks. 

In contrast to the preindustrial simula- 
tions, 20th-century simulations (30) show 
that solar irradiance changes alone warm the 
surface by ~0.25°C, but when ozone changes 



and climate feedbacks are included, the cool- 
ing drops to —0.1 9°C. With current atmo- 
spheric composition, ozone enhancement 
fh)m increased irradiance outweighs temper- 
ature-induced changes, leading to more mid- 
dle and upper stratospheric ozone (9), which 
dampens the net radiative forcing. Ozone's 
reversal from a positive (preindustrial) to a 
negative feedback supports results showing 
that solar forcing has been a relatively minor 
contributor to late 20th-century surface 
warming (7, 19, 31). However, ozone feed- 
backs remain important to the dynamical re- 
sponse associated with decadal and shorter 
term solar variability, which is initiated by 
upper stratospheric temperature changes, 
which are proportional to solar heating and 
hence to ozone (9). 

The AO/NAO response is dependent on 
the overall forced change in tropical and sub- 
tropical SSTs, translation of these changes 
into upper tropospheric temperature changes 
by hydrological feedbacks, and the resulting 
wind and planetary wave refraction changes. 
These processes vary between models and in 
particular are sensitive to the inclusion of a 
realistic stratosphere (22, 25), emphasizing 
the need for intermodel comparisons of sim- 



ilar experiments (32). 

The ECHAM3 GCM exhibits a similar 
spatial pattern of response to solar forcing, 
with warming over the NH continents and 
cooling over the North Atlantic and North 
Pacific (33). In that model, however, the sur- 
face temperature and AO sensitivity were 
much less than in our simulation. This is 
probably due to the lack of interactive ozone 
and the pooriy resolved stratosphere, two im- 
portant positive feedbacks in our model. 

The importance of SST changes to initiate 
the AO/NAO response is consistent with our 
fixed-SST solar cycle experiments (9, 22), 
whose response did not project strongly onto 
the AO/NAO, and with the weak empirical 
instantaneous climate sensitivity (Fig. 3, top). 
Present-day observations and modeling also 
support links between long-term tropical SST 
changes and the NAO (34,35). 

Surface temperatures in an energy bal- 
ance model (36) driven by volcanic forcing 
gave a poor correlation with proxies during 
the 17th and 18th centuries. Correlations 
between calculations of solar-induced cli- 
mate change and temperature proxies were 
quite good, however, although the impacts 
were very small. Inclusion of ozone chem- 
ical feedback would amplify the response, 
potentially reproducing observed ampli- 
tudes as well as variability. 

The GISS model results and empirical 
reconstructions both suggest that solar-forced 
regional climate changes during the Maunder 
Minimum appeared predominantly as a shift 
toward the low AO/NAO index. Although 
global average temperature changes were 
small, modeled regional cooling over the 
continents during winter was up to five times 
greater. Changes in ocean circulation were 
not considered in this model. However, given 
the sensitivity of the North Atlantic to AO/ 
NAO forcing (37), oceanic changes may well 
have been triggered as a response to the 
atmospheric changes (3S). Such oceanic 
changes would themselves further modify the 
pattern of SST in the North Atlantic (39) and, 
to a lesser extent, the downstream air temper- 
ature anomalies in Europe. 

These results provide evidence that rela- 
tively small solar forcing may play a signif- 
icant role in century- scale NH winter climate 
change. This suggests that colder winter tem- 
peratures over the NH continents during por- 
tions of the 15th through the 17th centuries 
(sometimes called the Little Ice Age) and 
warmer temperatures during the 12th through 
14th centuries (the putative Medieval Warm 
Period) may have been influenced by long- 
term solar variations. 



References and Notes 

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www.sciencemag.org SCIENCE VOL • MONTH 2001 



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40. Supplemental Web material is available on Science 
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VOL/ISSUE/PACE/DCI. 

41. Stratospheric climate modeling at GISS is supported 
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Analysis Program. G.A.S. and D.T.S. acknowledge the 
support of NSF grant ATM-00-02267. M.E.M. ac- 
knowledges support from the National Oceanic and 
Atmospheric Administration- and NSF-supported 
Earth Systems History program. We thank J. Lean for 
providing the solar irradiance reconstruction. 

13 July 2001; accepted 31 October 2001 



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